This section focuses on the adverse side of the food chain rather than the overall food chain. There is a complex web of events before food is eaten by humans. A food web might be more accurate than the more familiar food chain as most organisms eat from a diversity of food sources. The food chain is the process whereby nitrogen and carbon are synthesised to nutrients, simple and complex by microorganisms, seaplankton and plants. These are eaten by ruminants, vegetarian animals and fish which are in turn eaten by and carnivores, including humans.

The amount of carbon in the atmosphere relative to that in the earth and ocean is important. The balance between net primary use for synthesis of organic substances of carbon and subsequent decomposition is affected by a number of factors. Climate affects the accumulation of carbon and nitrogen in the earth and water. The store of carbon on earth and in water ( some 2000 Gtonnes ) is three times that in the atmosphere ( 700 Gtonnes) Water particularly the oceans contains dissolved carbon dioxide. Oceans circulate massive currents of water and hence transport large amounts of heat, fresh water and nutrients around the world and as discussed in the previous chapter are also regulators of climate and nutrition. Sea ice is a crucial boundary between the ocean and the atmosphere, which varies from ocean to ocean and even between the landlocked Arctic and the sea surrounded Antarctica. The thickness of the ice cap varies through the seasons but overall the ice cap reducing in thickness. As the ice thaws, the salt it contains sinks and affects the circulation of water in the oceans with further effects on climate and marine fixation of carbon and nitrogen. Warm climates increases carbon loss from the earth and decomposition. Industry is an important source of carbon dioxide emissions.

Photosynthesis, the decay of organic substances and circulating carbon dioxide have long term effects on the climate and carbon cycle.. Photosynthesis by terrestial vegetation accounts for half of the carbon that annually cycles between the earth and the atmosphere. There is an important contribution of forestry and farming practices to the overall distribution of carbon, to offset some of the carbon released by burning fossil fuels. The northern peat lands decay very slowly and remove carbon dioxide from the atmosphere faster than it has been released and consequently contain 20-30% of the earth’s carbon stock which is 60% of the atmospheric carbon pool. Cool conditions and poor air penetration of the soil reduces the decomposition of soil organic matter. Respiration by roots contributes a significant amount of carbon dioxide emission from the ground. Deep ploughing releases carbon from soil. Plant toxins

Plants may serve as poisons. Hemlock, a member of the parsley family, has long been known as a deadly poison. Most chemicals which are natural poisons in plants are known as ‘secondary’ products, produced by the plant only to form part of its defence mechanism against herbivores and pathogens. Plants do also produce certain ‘primary’ products such as amino acids, amines and organic acids e.g. oxalic acid which may enter the human food chain and cause poisoning. Humans have, through trial and error, learned to avoid certain poisonous plants as foods. Some plant poisons are minimised in their toxicity by cooking.

There is a system within cells (p-glycoproteins) which is found especially in the mucosa of the gastrointestinal tract, (oesophagus, stomach and colon) which is protective against toxic substances in plants, bacteria and fungi by pumping toxic substances from the cell. Drugs commonly used in cancer chemotherapy, against which resistance develops, have a common origin in that they are all products of plants, fungi and bacteria. Resistance appears to be related to the amount of p-glycoprotein 170 in the tumour cell. The multi drug resistance-1 gene associated with p-glycoprotein 170 is amplified in patients receiving drugs of plant or bacterial origin. This perhaps accounts for the fact that cancer in the oesophagus, stomach and colon are so resistant to cancer chemotherapy.

Proteinaceous substances, lectins, are found throughout plants, in seeds, bulbs, bark and leaves, acting as protection against unwanted predators. They are resistant to proteolysis and bind strongly to the brush border of the small intestinal epithelium resulting in increased growth of the small intestine. Bacterial colonisation of the small intestine can then occur. Lectins bind to specific sugars of the epithelial membrane, glycoconjugates.

Toxicants are grouped together by chemical classification, e.g. alkaloids, cyanogenic glycosides, amino acids, or by their effect, e.g. carcinogens and oestrogens.

Alkaloids

Alkaloids of which there are possibly 6000 contain nitrogen as part of a heterocyclic ring structure and are often bitter to taste. They are present in about 25% of all plant species. Alkaloids are an important cause of liver, lung and heart damage, neurological disorder and birth defects in livestock, to which alkaloids present a much greater risk than to humans, because of the plants and therefore toxins consumed as food. Some do, however, pass into the human food chain including solarium alkaloids and pyrrolizidine alkaloids.

Solanum alkaloid. The solanum alkaloids include sugar-based alkaloids (glycoalkaloids) found in potatoes, apples, egg plants, roots and leaves of tomatoes and sugar beet roots. These alkaloids have anticholinesterase properties. The poisoning effects include influenza-like symptoms, headache, nausea, fatigue, vomiting, abdominal pain and diarrhoea. The alkaloid concentration in healthy potatoes is in the order of 2–13 mg solanine/100 g fresh weight, predominantly in the potato skin. The level of solanine is increased in the parts of the potato, green after exposure to light, resulting in a dangerous concentration of 80–100 mg/100 g fresh weight (20 mg/100 g is the generally accepted upper limit of safety for solanine). Cooking the potato does not reduce solanine concentrations, as the solanine is stable at increased temperature.

Pyrrolizidine alkaloids. Some 3% of the flowering plants in the world, about 6000 species, contain pyrrolizidine alkaloids (senecio alkaloids) with about 250 different chemical structures. Pyrrolizidine alkaloid poisoning generally presents as hepatic disease with chronic liver failure and periportal fibrosis resulting in cirrhosis and veno-occlusive disease as the central veins of the liver are blocked by connective tissue. Pyrrolizidine poisoning usually occurs from contact with weeds contaminating food crops, including wheat or corn, harvested with the grain. The problem may be compounded by plants of Senecio and Crotalaira being used as folk medicine. They may also be taken in the form of herbal teas, e.g. gordolobo yerba. Flower stalks of Petasites japonicus, which are used as a cough medicine in Japan, also contain the pyrrolizidine alkaloid, petasitenine.

Aconites, the dried root stalks of plants in the Aconitium family are used as herbal medicines to treat rheumatism, neuralgia and cardiac complaints. They are prepared by boiling or soaking in water which hydrolyses the toxic alkaloids into less toxic derivatives. To avoid severe poisoning a limit of 1.5–3.0 g of cured aconitum root stalk is recommended.

Cyanogenic glycosides

These are synthesized by plants and contain sugar molecules and ?-hydroxynitriles (cyanohydrins). When these are degraded by plant enzymes, nitrile groups are eliminated as hydrogen cyanide which is toxic; 50–60 mg is an average fatal dose in humans. Cyanogenic glycosides have been detected in 110 plant families and over 2000 plant species. These include the plants identified in Table 4.3.

Cyanogenic glycoside poisoning is most seen in populations eating lima beans and cassava, though the concentration in lima varies markedly from the American white bean to the Puerto Rican small black. In cassava, however, cooking removes or destroys cyanogenic glycoside and the enzymes that cause the liberation of hydrogen cyanide. Linseeds, traditionally used as laxatives, are cyanogenic but toxic effects are unknown at traditional levels of intake, though altered processing and increased intake can lead to toxicity. The cyanide content varies from batch to batch (4–12 mmol/kg) and contains the cyanogenic glucoside as cassava. Poisoning from cyanogenic glycosides can also occur from the ingestion of bitter almonds, choke, cherry seeds and drinking tea made from peach leaves.

TABLE 4.3 Hydrogen cyanide (HCN) levels liberated from food crops containing cyanogenic glycosides Food Hydrogen cyanide (HCN) yield (mg/100 g) Bitter almond seed 290 Peach seed 160 Cassava leaves 104 Apricot seed 60 Lima bean Puerto Rico small black 400 American White 10

Amino acids and amines

Amino acids There are more than 200 amino acids but only 25 to 30 are universally incorporated into proteins or occur as intermediates in metabolism. Some uncommon amino acids interfere with metabolism and consequently are toxic. Seeds of the lathyrus species including chickling vetch, the flat-podded vetch and the Spanish vetchling contain lathyrogens, toxic amino acids which cause lathyrism. Lathyrism is characterized by muscular weakness and paralysis of the lower limbs and may be fatal. It is particularly prevalent in times of famine when there is reduced choice in food.

Another uncommon plant amino acid which is extremely poisonous is 3-methylenecyclopropylpropionic acid (hypoglycin A). This is found in the fruit of the tropical tree, Blighia sapida. This plant is poisonous when the fruit is unripe or when the inadequately cooked fruit is eaten. The consequences are severe vomiting, coma, acute hypoglycaemia and death within 12 hours. The toxicity is due to interference with the oxidation of fatty acids and, as a result, undernourished individuals are particularly vulnerable.

Excess ingestion of the selenium-containing amino acids, selenomethionine and selenocystine results in dermatitis, fatigue, dizziness and hair loss. Such amino acid poisoning results from the consumption of the nuts from the monkey nut tree, Lecythis olloria, found in Central and South America.

Amines These include serotonin, noradrenaline, tyramine, tryptamine and dopamine. These are very active pharmacologically, are potent vasoconstrictors and are found in bananas, plantains, pineapples, avocados, tomatoes and plums. While levels of these amines normally eaten are readily detoxified, excessive consumption of these plants may prove to be poisonous. Patients receiving the monoamine oxidase inhibitor group of antidepressants, who must avoid amine-rich diets are particularly vulnerable..

Glucosinolates

These are produced by the brassica plant family, including cabbage, kale, Brussel sprouts, cauliflower, broccoli, turnips, garden cress, water cress, radishes and horseradish, rape seed, brown, black and white mustard. There are more than 70 different glucosinolates, of which most of the crucifer plants contain several. Glucosinolates are responsible for the pungent flavours of horseradish and mustard and the characteristic flavour of turnip, cabbage and other related plants. They have been shown to interfere with thyroid function in experimental animals. Toxic effects arise from metabolic products formed from the action of the enzymes thioglucosidases, which break the glucosinolates into glucose, organic nitriles, isothiocyanates and thiocyanate ions. It is these latter two substances which modify thyroid function. It is, however, only at very high ingestion rates of raw plants, e.g. 500 g of raw cabbage daily for 2 weeks, that any effect on the thyroid gland is noticed.

Specific toxic effects

Carcinogens

• Safrole is a carcinogen found in several oils, including oil of sassafras, camphor and nutmeg. Safrole has been found in 53 plant species and in 10 plant families, and has been shown to produces liver cancer when sufficient is added to a rat diet. Safrole represents 75% of the weight of oil of sassafras, which was previously used as a flavouring agent in root beer. Black pepper contains small amounts of safrole and larger amounts of piprine, which has been shown to be carcinogenic to mice.

• Furanocoumarins are carcinogenic chemicals produced by celery, parsley and parsnip. However, the concentration in these plants is low but may increase in diseased plants. The most common furanocoumarins are psoralen, bergapten (5-methoxypsoralen) and zanthotoxin (8-methoxypsoralen). Cycasin is found in cycads which are important sources of starch for tropical and sub-tropical populations. Such compounds are capable of producing liver, kidney, intestinal and lung cancers in rats.

• Pyrrolizidine alkaloids have caused cancer in rodents, and human cancers have been reported from the use of herbal remedies containing these alkaloids.

• Stevioside is a very sweet glycoside from Stevia rebaudiana. Steviol is the aglycone resulting from bacterial hydrolysis, and has potential for carcinogenesis.

• Cycasin is a glycoside present in the nuts of Cycas circinalis. This is harmless as the glycone, but the ?-glycone methylazoxymethanol is carcinogenic. The ?-glucosidase which splits off the carcinogenic ?-glycone is present in tissues and in colonic bacteria.

Oestrogens At least 50 plants are known to contain chemicals which have oestrogenic activity, including carrots, soy beans, wheat, rice, oats, barley, potatoes, apples, cherry, plums and wheatgerm. Oestrogens are also present in vegetable oils such as cotton seed, sunflower, corn, linseed, olive and coconut oils. The oestrogenic activity rests in isoflavones, coumestans or resorcyclic acid lactones. It is doubtful if physiological effects would be elicited in humans by normal consumption of foods containing these weakly oestrogenic chemicals

Human female natural urinary oestrogens and excreted oral contraceptives can feminise male fish when there is sewage effluent to a river.

Hormone disrupters are chemicals which affect human or animal health by interfering with normal hormonal processes. They are mediated by hormone receptors and their effect may be at low dosages. However the response is variable and the differences in response may be due to genetic variation. Bisphenyl A, a chemical in plastics and glues functions as a relatively potent oestrogen in some circumstances.

Mutagens The cooking and processing of meat and fish at high temperatures results in heterocyclic amines with mutagenic and carcinogenic potential as judged by the Ames’ test.

Miscellaneous toxic agents

Beans The broad-bean, or lava bean, can produce acute haemolytic anaemia (favism) which is prevalent in Mediterranean countries, China and Bulgaria. The disease is characterized by nausea, dyspnoea, fever and chills and occurs 5–24 hours after broad-bean ingestion. Individuals who are susceptible to favism are deficient in glucose 6-phosphate dehydrogenase which also results in resistance to malaria.

Lentils Red lentils (Lensculinaris) are pulses which produce modest crops. Similar pulses from Vicia sativa are sometimes substituted in the diet of some populations. Cultivars of V. sativa may contain two neurotoxins, L-?-cyanoalanine and ?-L-glutamyl derivatives at a concentration of approximately 0.1 %, a level capable of being toxic to animals. Most, if not all, of the neurotoxins are lost if the seeds are soaked and soaking and cooking water is discarded. V sativa also contains pyrilidine glucoside. V. sativa and V. faba can also cause favism.

Myristicin This is a potent hallucinogenic chemical produced by dill, celery, parsley, parsnip, mint and also nutmeg. It is said that as little as 500 mg of raw nutmeg may produce psychoactive symptoms while 5–15 mg of powdered nutmeg may result in euphoria, hallucinations and a dreamlike feeling, followed by abdominal pain, depression and stupor.

Oxalates Oxalates may be produced endogenously by the metabolism of ascorbic acid or the amino acid glycine, which is also derived from the families Polygonaceae, Chenopodiacea, Portulacaceae and Fidoidaceae. Spinach contains 0.3–1.2%, rhubarb 0.2–1.3%, beet leaves 0.3–0.9%, tea 0.3–2% and cocoa 0.5–0.9%. The leaves of rhubarb are particularly rich in oxalic acid.

Gossypol This is the yellow colouring of cotton, Gossypium. It is found in the pigment glands of the leaves, stems, roots and seeds and may form 20–30% of the weight of the gland. When ingested, the result is depressed appetite and loss of body weight, cardiac irregularity and circulatory failure or pulmonary oedema. A major source of gossypol in the diet can be cotton seed oil, which may be found in salad oil, margarine and shortening. Gossypol has also been used in China with a 99% effectiveness as a male anti-fertility agent.

Diterpenoids The honey from wild rhododendrons (Rhododendron luteum and Rhododendron ponticum) may be poisonous due to the nectar containing toxic diterpenoids (grayanotoxins).

Microbial toxins

Microbial toxins are poisonous metabolites produced by bacteria, filamentous fungi, mushrooms and algae. Bacterial fungi grow commonly in food, competing for nutrients with animals and humans, as mushrooms and algae are themselves a form of food supply. Bacteria and fungi may proliferate within a food as it is stored or may proliferate on entering the gastrointestinal tract.

Bacterial toxins

Most illnesses caused by pathogenic microorganisms result from proliferation of pathogenic microorganisms in the host, usually in the gastrointestinal tract. In other cases, poisoning arises from the ingestion of toxins. Food-borne diseases usually cause gastrointestinal disturbance. Toxins produced by bacteria have different modes of action and have individual toxic characteristics (Table 4.4).

Some toxins produce the same type of cellular disorder and therefore the toxin is named after the specific action. Toxins that cause enteric disorders, e.g. cholera, salmonellosis and E. coli are called enterotoxins. Others, such as tetanus and botulinum, are neurotoxins. Bacterial toxins are usually either endotoxins or exotoxins. Endotoxins are released upon the disintegration or death of bacterial cells in the body. These produce specific toxic effects either in specific tissues or affecting the whole body. They are toxic when there is massive bacterial infection in the body and their effects may be due to an over-reaction of the host immune system. Exotoxins are special proteins excreted by toxigenic bacteria in various foods and are generally extremely poisonous and may be fatal, e.g. botulism, cholera and gastroenteritis.

Food is only the final link in a chain of infections and as a suitable medium may determine the degree to which a product is infected. For example cholera-causing organisms do not thrive in acid foods. Botulism is caused by food contaminated by strains of Clostridium botulinum. This highly lethal organism is the most poisonous known to man.

TABLE 4.4 Relative potency of toxins Toxin Lethal dose to mice (?g/kg body weight) Bacterial toxins 0.00003–1 Animal venoms 10–100 Algal toxins 10–1000 Mushroom toxins > 1000 Mycotoxins 1000–10 000 Mycotoxins Mycotoxins are highly poisonous compounds, of low molecular weight, produced by moulds or fungi which are contaminants of fruit and agricultural products. If mould growth occurs on any food there is the possibility of mycotoxin production which may persist long after the mould has disappeared. A large number of commonly consumed foods therefore may potentially contain mycotoxins. These include wheat, flour, bread, corn meal and popcorn, which may contain aflatoxin, ochratoxin, serigmatocystin, patulin, penicillic acid, deoxynivalenol or zearalenone. Peanuts and pecans may contain aflatoxins, ochratoxin, patulin and strigmatocystin. Apples and apple products may contain patulin. Cereal grains are a good substrate for toxin production, whereas seeds that are high in protein, soy beans, peanuts and cotton seeds support certain toxins but not others.

Growth of microorganisms in food will be influenced by moisture content, relative humidity, temperature, food composition, presence of competing microorganisms and fungal strain. The critical moisture content varies with the commodity. Storage fungi are primarily aspergilli and some penicillia. Variations in the moisture content of stored materials in different areas throughout storage bins (hot spots) allow fungal growth and toxic development. Other materials in the commodity, e.g. zinc, can also affect the ability of fungal growth and toxin production, e.g. aflatoxin and soya beans.

Significant food-borne mycotoxins These include aflatoxins, ochratoxin A, citrinin, patulin, penicillic acid, zearalenone, trichothecenes and alternaria toxins.

• Aflatoxins are produced by some strains of Aspergillus flavus and A. parasiticus. There are six main aflatoxins: B1, B2, G1, G2, M1, M2. Aflatoxin B, is a principal member of the aflatoxin family. Aflatoxin M is a metabolite of Bl, found in the milk of dairy cattle which have ingested mouldy feed and readily produces cancer of the liver. It is stable in raw milk and processed milk products, and is unaffected by pasteurization or processing into cheese or yoghurt. The widespread use of milk and milk products by children make this toxin of importance. Foods commonly contaminated with aflatoxins include peanuts, peanut oil, corn and beans. Aflatoxin B can kill poultry and domestic animals. Aflatoxins are potent liver toxins and carcinogenic in animals but, it appears from studies in the United States of otherwise fit people, not in humans. Other studies have shown that aflatoxin contamination of food correlates with the incidence of liver cancer in high-risk areas, such as south-east Asia and tropical Africa where malnourishment and viral hepatitis are endemic. The mortality rate from liver cancer among individuals infected with hepatitis B and who are anti-body positive is 10 times higher than in individuals who are antibody negative when eating small amounts of these infected materials. It has been suggested that 50% of liver cancer cases in Shanghai are related to aflatoxin exposure.

• Ochratoxins are produced by Aspergillus ochraceus and other Aspergilla species. They can contaminate corn, pork, barley, wheat, oats, peanuts, green coffee and beans. Ochratoxin A, which frequently occurs in wheat and barley, can cause kidney damage in rats, dogs and pigs and may cause kidney disease in humans. Only 2–7% of the ochratoxin A in barley is transmitted to beer during processing. Over 80% of ochratoxin A is destroyed on roasting of coffee and variable losses of ochratoxin A occur when baking with toxin-contaminated flour.

• Citrinin is a yellow compound produced by Penicillium and Aspergiculla species, and is a contaminant of yellow peanut kernels from damaged pods. It may be strongly nephrotoxic though in general is less toxic than ochratoxin A.

• Patulin is toxic to bacteria, mammalian cell cultures, higher plants and animals. Patulin, produced by a dozen Penicillium and Aspergiculla species, .is a principal cause of apple rot and a common pathogen on many fruits and vegetables. It is a contaminant of fruit juices world-wide, particularly apple juice. It is unstable in the presence of sulphydryl compounds and sulphur dioxide. When fruit juices are left to ferment more than 99% of the patulin is destroyed.

• Zearalenone is an oestrogenic compound which causes vulvovaginitis and oestrogenic responses in pigs. It is produced by Fusarium species and is found in moist corn in autumn and winter, is not very toxic and has not been implicated in human disease.

Mushroom toxins

There are thousands of mushroom species. In the United States there may be more than 5000. There are many species with very similar appearance and yet quite different tissues and cellular structure. Edible or poisonous species may differ quite radically in different environmental growth conditions. The poisonous Amanita muscarina comes in three colours, dark red, yellowish orange and white. These vary in intensity with age or exposure to sun and rain. The orange-capped toxic variety can be confused with the edible Acesarea. A characteristic feature of a species appearance may be changed by mechanical damage which may lead to errors in identity. There are many individual responses to the toxins which have resulted in conflicting reports regarding edibility in the literature. Responses vary with the number of mushrooms eaten, the preparation, length of cooking, age and the health of the individual as well as the amount of toxin present in the mushroom. There is no simple test for the toxicity of fungi.

Types of mushroom poisoning

These include: cytotoxic, haemotoxic, neurotoxic, hallucinogenic, gastrointestinal, disulfiram-like activity and carcinogenic.

Cytotoxic mushrooms

The most important toxins in this group are amatoxins and phallotoxins. Amatoxins are 10–20 times more toxic than phallotoxins and there is no known antidote. Phallotoxins are hepatotoxic whereas amatoxins are both strongly hepatotoxic and nephrotoxic. Fungi containing amanitin include the species Amanita galerina and A. conocybee. Among the most poisonous of mushrooms is Amanita phalloides: this has a large cap which is greenish brownish in tone; the smell is of raw potato; the taste, reported by survivors, is said to be quite good. The Galerina are small brown to buff mushrooms with moist, sticky caps, found on logs buried deep in moss and are equal in toxicity to A. phalloides.

Haemotoxic mushrooms

The Ear mushroom causes inhibition of blood clotting when eaten in sufficient amounts.

Haemotoxic mushrooms

Amanita muscarina and A. pantherina are important examples of neurotoxic mushrooms which cause increased salivation, lacrymation, sweating and severe gastrointestinal disturbances.

Hallucinogenic mushrooms

Two mushrooms, Psilocybe and Panaeolus may cause euphoria and excitement as well as muscle incoordination and weakness of arms and legs. Panic reactions may follow psychedelic visions of intense bright-coloured patterns and are associated with an inability to distinguish between fantasy and reality.

Gastrointestinal toxic mushrooms

These cause abdominal cramp, intense abdominal pain, nausea, vomiting and diarrhoea which may be incapacitating and may even cause death in children. A. agaricus includes the widely available mushrooms sold in supermarkets as well as a number of phenol-smelling yellow stainers. These mushrooms are variably edible or toxic, suggesting the existence of local mildly toxic forms.

Disulfiram-like mushrooms

These induce hypersensitivity to ethanol and when eaten with alcohol can cause severe flushing of the face, palpitations, tachycardia, nausea and vomiting. Disulfiram may be offered to alcoholics as an external deterrent to drinking.

Carcinogenic mushrooms

Laboratory assays have identified carcinogenic and mutagenic properties in mushrooms. These include the false morel gyromitrin and two common types, the common commercial supermarket mushroom, Agaricus bisporus and the Japanese forest mushroom, Cortinellus shiitake. The toxins responsible are unstable to heat.

1. In the food chain, nitrogen and carbon are converted to complex and essential nutrients by microorganisms, sea plankton and plants. There is a complex chain of events, which progresses from the inorganic chemicals in the soil to plants, which are ingested and digested by animals.

2. Food additives and preservatives are substances, either synthetic or natural, which are normally not regarded as food, but added in small amounts to influence the desirability of food or to preserve food. Additives are also used for enrichment and fortification of nutrients

3. Food-borne microbes or fungi are other non-nutrients in the human food chain. Antibiotics, pesticides, metals and fertilisers can enter the food chain and hence human food.

4. Industrial contaminants may leach out into the soil and be taken up by plants and enter the human food cycle.

5. Many plants, plant leaves, stalks and roots contain poisonous chemicals which may well occur to protect the plant from being eaten.

6. Toxins can be produced by bacteria, fungi, mushrooms and algae and are important causes of food-borne illness.

7. Food-borne microbes or fungi are other non-nutrients in the human food chain. Antibiotics, pesticides, metals and fertilisers can enter the food chain and hence human food.

8. Industrial contaminants may leach out into the soil and be taken up by plants and enter the human food cycle.

9. Many plants, plant leaves, stalks and roots contain poisonous chemicals which may well occur to protect the plant from being eaten.

10. Toxins can be produced by bacteria, fungi, mushrooms and algae and are important causes of food-borne illness.

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D’Mello, J.PF, Duffus, C.M. and Duffus, J.H. (1991) Toxic substances in crop plants. Royal Society of Chemistry, Cambridge.

European Parliament and Council Directive No. 95/2/EC (1995) Food additives other than colours and sweeteners. Official Journal of the European Communities, L61/1.

Kump LR ( 2000) What drives climate. Nature 408, 651-2

Post DM, Pace ML, Hairston NG (2000). Ecosystem size determines food-chain length in lakes. Nature. 406, 1047-9

Select Committee of the UK Parliament . House of Commons . Agriculture Fourth Report II . The food supply problem 2000

Winter, C.K., Seiber, J.N. and Muckton, C.F. (eds) (1990) Chemicals in the Human Food Chain, Van Nostrand Reinhold, New York.

Websites

The UK, European Union , World Health Organisation and other national website will give details of the contaminants of the food chain.

www.who.dk specific environmental hazards UK

www.environment.detr.gov.uk quality of water UK, Department of the Environment.